Nonlinear optical properties of certain materials can be employed to double or triple the frequency of electromagnetic radiation. For example, it has long been known that the frequency of lasers can be doubled by directing the beam through a quartz crystal. Several other inorganic materials, such as potassium dihydrogen phosphate, lithium niobate, cadmium sulfide, cadmium selenide, cadmium telluride, and cadmium germanium arsenide also have been the subject of nonlinear optical (NLO) research.
Recently, there has been considerable interest in development of organic and polymeric materials that can exhibit second and third order NLO phenomena. It is now known, for example, that second order nonlinear optical effects are exhibited only by materials that are noncentrosymmetric. Such organic and polymeric materials generally also require incorporation of suitable chromophores, such as azo dyes. Consequently, the variety of organic materials believed to be capable of second order NLO phenomena has been limited.
Third order NLO behavior, on the other hand, does not have the same symmetry requirements as that required to generate second order NLO effects. Among the most widely studied classes of polymers in the field of third order nonlinear optics is that of polydiacetylenes. A primary reason for studying third order nonlinear optical behavior of polydiacetylenes is that they are rigorously defined in terms of composition and chain sequence, and can often be synthesized in the form of macroscopic single crystals.
Possible applications for materials that exhibit third order nonlinear optical behavior include switching, amplification and multiplexing. In particular, third order nonlinear optical technology has been considered for many uses, such as incorporation into optical limiters to protect sensors and/or employment in advanced holographic techniques. Other potential applications include use as thermochromic sensors, photoreceptors, materials for optical storage, photoconductors for laser printers, and as photochemotherapeutic agents, among others. However, the absorption maxima exhibited by known polydiacetylenes has limited their utility.
One attempt to increase the wavelength at which crystallized polydiacetylenes exhibit absorption maxima is to incorporate sulfur-containing side chain substituents, such as alkylthio or arylthio groups. It is believed that the longer wavelength at which excitonic absorption maxima is observed is a consequence of stronger electron donation by alkylthio groups to the polymer chain of the polydiacetylene. However, such polymers generally must be formed by solid-state polymerization techniques that include ultraviolet or gamma ray irradiation. These techniques typically are difficult and expensive to conduct.
Therefore, a need exists for polydiacetylenes that exhibit absorption maxima at wavelengths greater than known polydiacetylenes, e.g., in the visible range, that overcome or minimize the above-mentioned problems.